Eclipse elimination by monitoring the pixel signal level

ABSTRACT

An anti-eclipse circuit for an imaging sensor monitors the photo signal level output by a pixel to determine whether the photo signal corresponds to the pixel being operated at a saturated state. If so, there is a risk that the pixel may be susceptible to an eclipse distortion. When the pixel is detected as being operated in a saturated state, the anti-eclipse circuit pulls up the reset signal level previously stored in a sample and hold circuit to an appropriate voltage level in order to prevent an eclipse distortion from arising.

FIELD OF INVENTION

The present invention is a continuation of U.S. patent application Ser.No. 12/869,821, filed Aug. 27, 2010, which is a continuation of U.S.patent application Ser. No. 11/125,097, filed May 20, 2005, now U.S.Pat. No. 7,872,682, issued Jan. 18, 2011, the disclosures of which areincorporated by reference in their entireties.

The present invention relates generally to pixel architectures forsemiconductor imagers. More specifically, the present invention relatesto an anti-eclipse system for image sensors.

BACKGROUND OF THE INVENTION

FIG. 1 is an illustration of a conventional four transistor (4T) pixel100. The pixel 100 includes a light sensitive element 101, shown as aphotodiode, a floating diffusion node C, and four transistors: atransfer transistor 111, a reset transistor 112, a source followertransistor 113, and a row select transistor 114. The pixel 100 accepts aTX control signal for controlling the conductivity of the transfertransistor 111, a RST control signal for controlling the conductivity ofthe reset transistor 112, and a ROW select signal for controlling theconductivity of the row select transistor 114. The voltage at thefloating diffusion node C controls the conductivity of the sourcefollower transistor 113. The output of the source follow transistor 113is presented at node B when the row select transistor 114 is conducting.

The states of the transfer and reset transistors 111, 112 determinewhether the floating diffusion node C is coupled to the light sensitiveelement 101 for receiving a photo-generated charge generated by thelight sensitive element 101 following a charge integration period, or asource of pixel power VAAPIX from node A during a reset period.

The pixel 100 is operated as follows. The ROW control signal is assertedhigh to cause the row select transistor 114 to conduct. At the sametime, the RST control signal is asserted high while the TX controlsignal is asserted low. This couples the floating diffusion node C tothe pixel power VAAPIX at node A, and resets the voltage at node C tothe pixel power VAAPIX. The pixel 100 outputs a reset signal Vrst atnode B. As will be explained in greater detail below in connection withFIG. 2, node B is typically coupled to a column line 215 (FIG. 2) of animager 200.

After the reset signal Vrst has been output, the RST control signal isasserted low. The light sensitive element 101 is exposed to incidentlight and accumulates charge based on the level of the incident lightduring a charge integration period. After the charge integration period,the TX control signal is asserted. This couples the floating diffusionnode C to the light sensitive element 101. Charge flows through thetransfer transistor 111 and diminishes the voltage at the floatingdiffusion node C. The pixel 100 outputs a photo signal Vsig at node B.The reset and photo signals Vrst, Vsig are different components of theoverall pixel output (i.e., Voutput=Vrst−Vsig), which is typicallyprocessed by an imager 200 (FIG. 2) as explained in greater detailbelow.

FIG. 2 is an illustration of an imager 200 that includes a plurality ofpixels 100 forming a pixel array 201. Due to space limitations the pixelarray 201 is drawn as a 4 row by 4 column array. One skilled in the artwould recognize that in most imagers 200 the pixel array 201 wouldordinarily include many more rows and columns, and thus, many morepixels 100.

The imager 200 also includes row circuitry 210, column circuitry 220, adigital conversion circuit 230, a digital processing circuit 240, and astorage device 250. The imager 200 also includes a controller 260. Therow circuitry 210 selects a row of pixels 100 from the pixel array 201.The pixels 100 in the selected row output, at different times, theirreset and pixel signals Vrst, Vsig to the column circuitry 220, viacolumn lines 215. The column circuit 220 samples and holds the reset andpixel signals Vrst, Vsig. The column circuitry 220 also forms an analogpixel output signal Vpixel from the difference Vrst−Vsig, and outputsthe Vpixel signal on lines 216 to the digital conversion circuit 230.

Now referring to FIG. 3, it can be seen that the column circuitry 220comprises a plurality of analog pixel processing circuits 221 and aplurality of corresponding load circuits 310. Each column line 215 iscoupled, in parallel at node D, to a respective analog processingcircuit 221 and a respective load circuit 310. Each analog pixelprocessing circuit 221 accepts the reset and pixel signals Vrst, Vsigoutput from a pixel at different times on column line 215, and forms ananalog pixel signal Vpixel as the difference between the reset and pixelsignals Vrst, Vsig (i.e., Vpixel=Vrst−Vsig). The signal Vpixel is outputon line 216.

FIG. 4 is a more detailed illustration of a single analog pixelprocessing circuit 221, its associated column and output lines 215, 216and load circuit 310. The analog pixel processing circuit 221 includes afirst signal path SP1 for sampling and holding a reset signal Vrst and asecond signal path SP2 for sampling and holding a photo signal Vsig. Thesampled and held Vrst, Vsig signals are provided to a gain stage 450,which outputs the pixel signal Vpixel on line 216. Additionally, theanalog processing circuit 221 further includes switches 431, 432, and433.

The first signal path SP1 includes switch 421, capacitor 441, and switch434. The state of switch 421 is controlled by the sample and hold reset(SHR) control signal, which is asserted high when a pixel is outputtingthe reset signal Vrst on line 215. The SHR control signal is assertedlow if the pixel is not outputting a reset signal Vrst.

The second signal path SP2 includes switch 422, capacitor 442, andswitch 435. The state of switch 421 is controlled by the sample and holdsignal (SHS) control signal, which is asserted high when a pixel isoutputting the photo signal Vsig on line 215. The SHS control signal isasserted low if the pixel is not outputting a photo signal Vsig.

The circuit 221 operates as follows. First, before a pixel coupled toline 215 outputs either the reset or photo signals Vrst, Vsig, thecapacitors 441, 442 must be set to a known state. Thus, switches 421,422, 432, 433, 434, and 435 are each opened, while switch 431 is closed.This equalizes the charges on the sides of capacitors 441, 442 closestto node D. Switches 432, 433 are then closed, to couple the sides ofcapacitors 441, 442, closest to gain stage 450 to a clamp voltage Vcl.Switches 431, 432, 433 are then opened.

The pixel coupled to output line 215 then outputs a reset signal Vrst online 215. The SHR control signal is asserted high while the SHS controlsignal is asserted low. This combination of the states of the SHR andSHS control signals causes switch 421 to close while maintaining switch422 in an open state, thereby coupling only the first signal path SP1 tonode D. The reset signal Vrst output by the pixel causes the chargelevel of capacitor 441 to change. Once the pixel has completedoutputting the reset signal Vrst, the SHR control signal is assertedlow, causing switch 421 to open, thereby decoupling the capacitor 441from node D.

The pixel coupled to output line 215 then outputs a photo signal Vsig online 215. The SHS control signal is asserted high while the SHR controlsignal is asserted low. This combination of the states of the SHR andSHS control signals causes switch 422 to close while maintaining switch421 in an open state, thereby coupling only the second signal path SP2to node D. The photo signal Vsig output by the pixel causes the chargelevel of capacitor 442 to change. Once the pixel has completedoutputting the photo signal Vsig, the SHS control signal is assertedlow, causing switch 422 to open, thereby decoupling the capacitors 442from node D.

Switches 434 and 435 are then simultaneously closed, which couples thegain stage 450 to capacitors 441, 442. The gain stage 450 produces ananalog pixel signal Vpixel equal to the difference Vrst−Vsig. The analogpixel signal Vpixel is output on line 216.

FIG. 5. is a more detailed illustration of the load circuit 310. Theload circuit 310 is comprised of transistors 311 and 312, coupled inseries by their sources and drains, between node D and a source ofground potential. The gate of transistor 311 is coupled to theVLN_enable control signal, which is used to switch transistor 311between an “on” and an “off” state. The gate of transistor 312 iscoupled to the VLN_bias control signal to control the conductivity oftransitor 312 to a predetermined level.

The pixel 100 (FIG. 1) is susceptible to a type of distortion known aseclipsing. Eclipsing refers to the distortion arising when a pixeloutputs a pixel signal corresponding to a dark pixel even though brightlight is incident upon the pixel. Eclipsing can occur when a pixel isexposed to bright light, as the light sensitive element 101 can producea large quantity of photogenerated charge. Once the level of theincident light exceeds a certain threshold, the light sensitive element101 becomes saturated and has generated a maximum amount of charge. Aneclipse condition can occur if the light sensitive element 101 producesso much charge that during the time between the falling edge of the RSTcontrol signal and the falling edge of the SHR control signal (i.e.,when the transfer transistor 111 is set to a non-conducting state) atleast some of the photo-generated charges spill over the transfertransistor 111 and make their way to the floating diffusion node C. Thisdiminishes the reset voltage at the floating diffusion node and cancause the pixel 100 to output an incorrect (i.e., diminished voltage)reset signal Vrst. This, in turn, can cause the reset and photo signalsVrst, Vsig to be nearly the same voltage. For example, the photo andreset signals Vrst, Vsig may each be approximately 0 volts. The pixeloutput signal Vpixel which equals (Vrst−Vsig), can therefore becomeapproximately 0 volts, which corresponds to an output voltage normallyassociated with a dark pixel.

An anti-eclipse circuit can be used to mitigate against the effect ofeclipsing. Conventional anti-eclipse circuits detect the presence of aneclipse condition by monitoring the voltage level of the reset signaland determining if that voltage level is abnormally low. If so, thereset signal can be pulled up to the proper level by clamping the columnoutput line to a voltage source. The proper voltage for the voltagesource is the normal reset signal voltage level. Unfortunately, thisvoltage varies from imager to imager because the voltage is sensitive tosemiconductor process variations. As a result, the voltage source istypically a controllable voltage source, such as a transistor having asource/drain coupled to a power supply voltage and a gate coupled to acontrol signal, typically designated as the AE_voltage bias signal. Postmanufacturing calibration could be done to set the AE_voltage biassignal to a proper level to permit the anti-eclipse circuit to pull thereset signal to the proper voltage when an eclipse condition isdetermined. Accordingly, there is a need and desire for an anti-eclipsecircuit, which is not dependent upon monitoring the voltage level of thereset signal, and which can operate without requiring calibration.

SUMMARY OF THE INVENTION

Exemplary embodiments of the method and apparatus of the presentinvention provide an anti-eclipse circuit for an imager. Theanti-eclipse circuit permits a pixel to initially output a reset signal,which is sampled-and-held. Subsequently, when the pixel outputs a photosignal, which is also sampled-and-held. While the pixel is outputtingthe photo signal, the voltage level of the photo signal is monitored todetermine whether the light sensitive element for producingphoto-generated charges is saturated. If so, the pixel may besusceptible to an eclipse condition. Accordingly, the anti-eclipsecircuit causes the previously sampled reset signal level to be pulled upto a proper voltage level, thereby ensuring that the reset signalvoltage used for generating the analog pixel voltage is at a correctvoltage level, thereby avoiding an eclipse condition.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of exemplaryembodiments of the invention given below with reference to theaccompanying drawings, in which:

FIG. 1 illustrates a conventional pixel;

FIG. 2 illustrates an imager utilizing the pixel of FIG. 1;

FIG. 3 illustrates column circuitry from the imager of FIG. 2;

FIG. 4 illustrates a portion of the column circuitry, including theanalog processing circuit, in greater detail;

FIG. 5 illustrates the load circuit portion of the column circuitry ingreater detail;

FIG. 6 illustrates a column circuit in accordance with an exemplaryembodiment of the present invention;

FIG. 7 illustrates a portion of the column circuitry of FIG. 6 ingreater detail; and

FIG. 8 illustrates a system incorporating an imager having the circuitsof FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

Now referring to the drawings, where like reference numerals designatelike elements, there is shown in FIG. 6, column circuitry 220′incorporating the anti-eclipse system of the present invention.

As illustrated, each column line 215 is associated with a processingblock 700. Each column line 215 is used to provide to its associatedprocessing block 700 a reset signal Vrst and a pixel signal Vsig (atdifferent times). The processing block 700, as described in greaterdetail below, produces an analog pixel signal Vpixel, which is protectedfrom eclipse distortion on line 216.

FIG. 7 is a more detailed illustration of processing block 700 of FIG.6. The column line 215 is respectively coupled in parallel at node D vialines 532, 533, and 534 to a photo signal monitor circuit 500, a loadcircuit 310, and an analog processing circuit 221′. The photo signalmonitor circuit 500 is also directly coupled to the analog processingcircuit 221′ via line 531. Additionally, the photo signal monitorcircuit 500 is also coupled via lines 621, 622 to a helper circuit 600.

The helper circuit 600 generates the AE_voltage bias signal, which issupplied to the photo signal monitor circuit 500 via line 621. Thehelper circuit 600 includes a load 510 and transistors 610, 620, 630,and 640, which are respectively coupled in series via their sources anddrains, between a source of power VAAPIX and ground potential. Asillustrated in FIG. 7, exemplary embodiments of the load circuit 510include a PMOS transistor 511 having its gate tied to the groundpotential or an NMOS transistor 512 configured to operate as a diode. Inone exemplary embodiment, each of the transistors 610, 620, 630, and 640are NMOS transistors. Transistor 610 is configured as illustrated tooutput the AE_voltage bias signal on line 621. The gate of transistor620 is coupled to the SHS control signal via line 622. The gates oftransistor 630 and 640 are respectively supplied the VLN_enable andVLN_bias control signals used for enabling and controlling the operationof the load circuit 310 (FIG. 5).

Transistors 630 and 640 are preferably fabricated such that they can becharacterized as having a reduced width-to-length (W/L) ratio ascorresponding transistors 311 and 312 of the load circuit 310 (FIG. 3).Transistors 630 and 640 therefore have a higher overdrive (Vgs) voltagethan transistors 311 and 312. This higher overdrive voltage ensures thatthe current source always stays in saturation.

The helper circuit 600 operates as follows. During any time when thepixel coupled to line 215 is not outputting a photo signal Vsig, atleast one of control signals SHS and VLN_enable will be asserted low,thereby causing the AE_voltage bias signal to be at VAAPIX. However,when the pixel coupled to line 215 is outputting the photo signal Vsig,both control signals SHS and VLN_enable will be asserted high, causingthe voltage level of the AE_voltage bias signal to be lower in voltagethan VAAPIX. The degree by which the AE_voltage bias signal voltagelevel is lower than the VAAPIX voltage is based on the voltage level ofthe VLN_bias control signal and the narrower width-to-length ratios (andthus the higher overdrive voltages) of transistors 630 and 640.

The photo signal monitor circuit 500 comprises a load 510, a firsttransistor 521, and a second transistor 522. The load 510 and thetransistors 521, 522 are connected in series, as shown in FIG. 7,between a source of pixel power VAAPIX and node D. Additionally, line531 is coupled between load 510 and transistor 521. Line 531 outputs theRESET_pullup control signal, which as described below is supplied to theanalog processing circuit 221′.

Now also comparing FIG. 7 with FIG. 4 (illustrating the conventionalanalog processing circuit 221), it can be seen that the analogprocessing circuit 221′ (FIG. 7) includes all of the components of theconventional analog processing circuit 221 (FIG. 4). The analogprocessing circuit 221′, however, includes an additional transistor 460.In one exemplary embodiment, the additional transistor 460 is a PMOStransistor having one source/drain coupled to a source of pixel powerVAAPIX and another source/drain coupled between switch 421 and capacitor441. The gate of transistor 460 is coupled to line 531 to receive theRESET_pullup control signal. Thus, if the RESET_pullup control signal isasserted high, the transistor 460 is non-conductive, and does not haveany affect on the charge stored on capacitor 441. However if theRESET_pullup control signal is asserted low, the transistor 460 becomesconductive, thereby coupling capacitor 441 to pixel power VAAPIX viatransistor 460, and changing the charge level of capacitor 441.

The invention operates as follows. First, before any pixel signals areprocessed, the charge level of capacitor 441 (for sampling and holdingthe reset signal Vrst) and capacitor 442 (for sampling and holding thephoto signal Vsig) are set to a predetermined state. Since the pixelcoupled to line 215 is not outputting either a photo signal or a resetsignal at this time, both the SHR and SHS control signals are assertedlow. Additionally, the VLN_enable control signal is also asserted low.

In circuit 600, both transistors 620 and 630 are set to a non-conductingstate respectively via control signal SHS and VLN_enable. As a result,the AE_voltage bias signal is set to VAAPIX.

In circuit 500, transistor 522 is set to a non-conducting state. As aresult, the RESET_pullup control signal is asserted high.

In circuit 221′, the low asserted SHR and SHS control signals setswitches 421 and 422 to an open state. Additionally, switches 432, 433,434, and 435 are also set to an open state, while switch 431 is set to aclosed state. The RESET_pullup control signal is asserted high, therebycausing PMOS transistor 460 to become non-conductive. Thus, the platesof capacitors 441, 442 nearest to switch 431 are coupled to each other,thereby equalizing their charges levels. Switches 432, 433 are then setto a closed state thereby coupling the plates of capacitors 441, 442closest to gain stage 450 to a clamp voltage Vcl. After a predeterminedtime, switches 431, 432, 433 are set to an open state and the charges oncapacitors 441, 442 have been initialized to a known predeterminedstate.

Second, when the pixel outputs the reset signal, the voltage level ofthe reset signal is sampled and held by capacitor 441 when SHR isasserted high. Since the pixel is outputting a reset signal on line 215,the SHR and VLN_enable control signals are asserted high, while the SHScontrol signal is asserted low.

In circuit 600, transistor 620 is set to a non-conducting state becausethe SHS control signal is asserted low. Accordingly, the helper circuit600 sets AE_voltage at VAAPIX.

In circuit 500, transistor 522 is set to a non-conducting state becausethe SHS control signal is asserted low. As a result, the circuit 500outputs a high RESET_pullup voltage.

In circuit 221′, switch 421 is set to a closed state by the high SHRcontrol signal, while switch 422 is set to an open state by the low SHScontrol signal. During this time, switches 431, 432, 433, 434, 435 areeach in the open state. The high RESET_pullup voltage sets transistor460 to a non-conductive state. As a result, the reset signal Vrst iscoupled to, and charges capacitor 441.

Next, when the pixel stops outputting the reset signal Vrst on line 215,the SHR and VLN_enable control signals are asserted low.

In circuit 600, transistor 620 is still set to a non-conducting statebecause the SHS control signal is still asserted low. Thus, circuit 600still outputs the AE_voltage bias signal at the VAAPIX voltage level.

In circuit 500, transistor 522 is still set to a non-conducting statebecause the SHS control signal is still asserted low. Thus, the circuit500 continues to output a high RESET_pullup control signal.

In circuit 221′, the low SHR control signal causes switch 421 to be setto an open state. The high RESET_pullup control signal maintains thetransistor 460 in a non-conducting state. As a result, the previouslysampled reset signal Vrst is now held in capacitor 441.

When the pixel outputs a photo signal Vsig on line 215, the SHS andVLN_enable control signals are asserted high, while the SHR controlsignal is asserted low.

In circuit 600, each one of transistors 610, 620, 630, and 640 areconducting. The voltage level of the AE_voltage bias signal becomeslower than VAAPIX and is dependent upon the voltage level of theVLN_bias control signal and the threshold voltages of transistors 610,620, 630, and 640.

In circuit 500, the amount of current flowing through load 510 andtransistors 521 and 522 is dependent upon the voltage level of the photosignal Vsig. Under normal circumstances, no current flows through thecircuit 500, thereby maintaining the voltage of RESET_pullup at a highvoltage. As the pixel is exposed to brighter and brighter light, thesignal voltage at the gate of the source follower of the pixeldiminishes. In one exemplary embodiment, the pixel begins to saturate asthe photo signal approaches 0.8 volts. At this point, no current flowsthrough circuit 500. By the time the photo signal approaches 0.4 volt,the circuit 500 is conductive and becomes more conductive as the photosignal level continues to drop. Once the circuit 500 becomes conductive,the voltage level of the RESET_pullup control signal begins to drop. Asdiscussed below in greater detail in connection with circuit 221′, thisbegins to charge capacitor 441 with an alternate reset signal throughtransistor 460.

In circuit 221′, the high SHS control signal sets switch 422 to a closedstate while the low SHR control signal sets switch 421 in an open state.This permits the photo signal Vsig to be sampled by capacitor 442.

If circuit 500 produces a high RESET_pullup control signal, transistor460 remains non-conducting and the previously sampled reset signal Vrstremains unaltered as stored in capacitor 441. However, if circuit 500produces a RESET_pullup control signal which causes transistor 460 tobecome conductive, the previously stored reset signal Vrst is altered bycharging capacitor 441 with voltage source VAAPIX via transistor 460.The charging rate is dependent upon the conductivity of the transistor460, which is based on the voltage level of the RESET_pullup controlsignal.

When the pixel finishes outputting the photo signal, control signals SHSand VLN_enable are each asserted low.

In circuit 600, both transistors 620 and 630 become non-conductive,thereby setting the AE_voltage bias signal to the VAAPIX voltage level.

In circuit 500, transistor 522 becomes non-conductive, thereby assertingRESET_pullup at the high level.

In circuit 221′, the high RESET_pullup signal sets transistor 460 to thenon-conductive state. Switch 422 is opened. By this time, the photosignal Vsig is sampled and held by capacitor 442. If the power supplyVAAPIX never charged capacitor 441, the originally sampled and heldreset signal Vrst is stored in capacitor 441. However, if the powersupply was used to charge capacitor 441, that indicates that the photosignal output was so diminished in voltage that there was a significantrisk that the originally sampled reset signal was subjected to aneclipse distortion. For this reason, the originally sampled reset signalis altered by charging capacitor 441 with the power supply VAAPIX.

The present invention is therefore directed to an anti-eclipse circuitwhich cooperates with the sample and hold circuit for sampling andholding the reset and photo signals. When a pixel is outputting a resetsignal, that reset signal is initially sampled and held. Then, when thepixel is outputting the photo signal, the voltage level of the photosignal is used to determine whether the incident light upon the pixelsignificantly exceeds the saturation limit of the pixel. If so, there isa risk of an eclipse, and the previously sampled and held reset signalis further charged to normalize the reset signal sample.

FIG. 8 illustrates a processor based system 800. The system 800 isexemplary of a digital system having an imaging device. Without beinglimited, system 800 could be a part of a computer system, camera,scanner, machine vision system, vehicle or personal navigation system,portable telephone with camera, video phone, surveillance system, autofocus system, optical tracking system, image stabilization system,motion detection system, or other system having an imaging function.System 800, for example, a camera, generally comprises a bus 820.Coupled to the bus 820 are a processor, such as CPU 802, a memory, suchas a RAM 804, a removable memory 814, an I/O device 806, and an imager200 including the circuit 700 (FIG. 7).

It should be appreciated that other embodiments of the invention includea method of manufacturing the circuit 700. For example, in one exemplaryembodiment, a method of manufacturing an anti-eclipse circuit includesthe steps of providing, over a portion of a substrate corresponding to asingle integrated circuit, at least a plurality of pixels 100, andcolumn circuitry 220′ including circuits 700. The pixels 100, columncircuitry 220′, and circuits 700 can be fabricated on a same integratedcircuit using known semiconductor fabrication techniques.

While the invention has been described in detail in connection with theexemplary embodiments, it should be understood that the invention is notlimited to the above disclosed embodiments. Rather, the invention can bemodified to incorporate any number of variations, alternations,substitutions, or equivalent arrangements not heretofore described, butwhich are commensurate with the spirit and scope of the invention.Accordingly, the invention is not limited by the foregoing descriptionor drawings, but is only limited by the scope of the appended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. An anti-eclipse circuit for an imager,comprising: a signal storing circuit connected to a column line and forstoring a reset signal in a storage element at a first time and forstoring a photo signal at a second time, said signal storing circuitcomprising: a replacement circuit for increasing a level of said storedreset signal in the storage element when a voltage level of said photosignal, as detected on said column line, is less than a predeterminedthreshold.
 2. The anti-eclipse circuit of claim 1, further comprising acontrol line coupled to the replacement circuit.
 3. The anti-eclipsecircuit of claim 2, wherein the replacement circuit is configured toincrease the level of said stored reset signal in response to a controlsignal on the control line.
 4. The anti-eclipse circuit of claim 3,further comprising a detection circuit for asserting said control signalin a first state when said voltage level of said photo signal is lessthan the predetermined threshold, as determined by the detectioncircuit.
 5. The anti-eclipse circuit of claim 4, wherein said resetsignal is stored in a first sample and hold circuit during said firsttime and said photo signal is stored in a second sample and hold circuitduring said second time.
 6. The anti-eclipse circuit of claim 5, whereinthe first and second sample and hold circuits are each coupled to thecolumn line.
 7. The anti-eclipse circuit of claim 6, wherein said firstsample and hold circuit includes a first sample capacitor.
 8. Theanti-eclipse circuit of claim 7, wherein said replacement circuitcomprises a switch connected in series between said first samplecapacitor and a power source, said switch also being coupled to saidcontrol line, and configured to maintain a closed state only when saidcontrol signal is asserted in said first state.
 9. An imager,comprising: an array of pixels for producing respective reset signalsand photo signals; a column circuit, coupled to said array of pixels viaa plurality of column lines, for selecting a row of pixels from saidarray for processing, said column circuit comprising: a plurality ofsignal storing circuits, each of said signal storing circuits connectedto a column line and for storing said reset signal produced by arespective pixel in a storage element at a first time, and for storingsaid photo signal produced by said respective pixel at a second time,each of said signal storing circuits comprising: a replacement circuitfor increasing a level of said stored reset signal in the storageelement when a voltage level of said photo signal, as detected on saidcolumn line, is less than a predetermined threshold.
 10. The imager ofclaim 9, wherein each signal storing circuit further comprises a controlline coupled to the replacement circuit.
 11. The imager of claim 10,wherein the replacement circuit is configured to increase the level ofsaid stored reset signal in response to a control signal on the controlline.
 12. The imager of claim 11, wherein each signal storing circuitfurther comprises a detection circuit for asserting said control signalin a first state when said voltage level of said photo signal is lessthan the predetermined threshold, as determined by the detectioncircuit.
 13. The imager of claim 12, wherein said reset signal is storedin a first sample and hold circuit during said first time and said photosignal is stored in a second sample and hold circuit during said secondtime.
 14. The imager of claim 13, wherein the first and second sampleand hold circuits are each coupled to the column line.
 15. The imager ofclaim 14, wherein said first sample and hold circuit includes a firstsample capacitor.
 16. The imager of claim 15, wherein said replacementcircuit comprises a switch connected in series between said first samplecapacitor and a power source, said switch also being coupled to saidcontrol line, and configured to maintain a closed state only when saidcontrol signal is asserted in said first state.